They need food and water after they wake up. Unless they have light, they will not eat in the dark.
I'd put the roosts up pretty quickly. My brooder raised chicks usually do not start roosting until they are 10 to 12 weeks old, but Mama usually starts taking the broody raised chicks to roost around 4 weeks old. If you read other people's experience, that timing can vary all over the place. Your experience will probably be different than mine.
My brooder raised chicks start playing and perching on the roosts at a fairly young age, but I lay a tree limb at an angle up to the roosts so they can walk up that tree limb. But they prefer to sleep in a huddle on the floor until they are older.
Chickens can be creatures of habit. You want them to get in the habit of sleeping on the roosts whenever they are ready to roost instead of getting in the habit of sleeping somewhere else.
I suggest you do not put the nest boxes in there or else leave the nest boxes closed where they cannot get into them until they are in the habit of sleeping on the roosts. When they are transitioning from sleeping on the floor to sleeping on the roosts, I have had some try to sleep in the nest boxes. If you were sure of the age, I'd suggest you open the nest boxes at 16 weeks, but since you are not sure, I'll just say after they are in the habit of sleeping on the roost. And make sure the roost is higher than the nest boxes.